Polyolefins, such as polypropylene (PP) and polyethylene (PE), are plastics that are difficult to coat. The problem is bad adhesion due to low surface tension in combination with the evaporation of paraffin. Flame treating is often used to improve the wetting and adhesion properties of polymer surfaces in general and of polyolefin surfaces in particular. The enhanced wetting property results in improved coatability and adhesion of materials such as pressure-sensitive adhesives, primers and low-adhesion release coatings.
Flame treaters ordinarily use premixed flames, i.e., the fuel and oxidizer are thoroughly mixed prior to combustion and the rate of combustion is controlled by the rate of chemical reaction that occurs in the flame. In a premixed flame, the luminous region is that portion of the flame where the rise in temperature is the greatest and where much of the reaction and heat release occurs. During a flame-treating process, one side of a polymer article is passed in close proximity to a flame, while the other side of the polymer surface may pass over a cooled support, e.g., a cooled drum, to minimize heat distortion.
Flames are commonly described in terms of two characteristics: the flame power and the molar ratio of oxidizer to fuel. The flame power is the product of the volume of fuel burned per unit time and the heat content of the fuel. Typical units for the flame power are W or Btu/hr. In flame treating, the flame power can be normalized to account for the dimensions of the burner, leading to units such as W/cm2 or Btu/hr-in2.
The exact ratio of oxidizer to fuel needed for complete combustion is known as the stoichiometric ratio. For example, the exact amount of dry air necessary for the complete combustion of methane is 9.55 volumes per volume of methane; so the stoichiometric ratio for an air:methane flame is 9.55:1. The equivalence ratio is defined as the stoichiometric oxidizer:fuel ratio divided by the actual oxidizer:fuel ratio. For fuel-lean (or oxidizing) flames, there is more than the stoichiometric amount of oxidizer and so the equivalence ratio is less than one. For oxidizer:fuel mixtures at the stoichiometric ratio, the equivalence ratio is equal to one. For fuel-rich systems, the equivalence ratio is greater than one.
Virtually all industrial flame treaters use a premixed laminar (as opposed to turbulent) flame with air as the oxidizer and a gaseous hydrocarbon as a fuel. Typical hydrocarbon fuels comprise hydrogen, natural gas, methane, ethane, propane, butane, ethylene, liquefied petroleum gas, acetylene, or blends thereof, and city gas, which is often composed of a mixture of carbon dioxide, carbon monoxide, hydrogen, methane, and nitrogen. Halogen and halogen-containing compounds have also been disclosed as oxidizer:fuel mixture additives to increase the adhesivity of polyolefin articles to subsequent coatings. The flaming step is usually automated.
However, flame treating a polymeric article can take a long time and can be expensive. It can also induce process variabilities, which in turn can lead to a higher rate of defective parts. In addition, the flaming process can generate design constraints, because parts have to be treated homogeneously.